Apparatus and process for bulk wet etch with leakage protection

ABSTRACT

When using hot alkaline etchants such as KOH, the wafer front side, where various devices and/or circuits are located, must be isolated from any contact with the etchant. This has been achieved by using two chambers that are separated from each other by the wafer that is to be etched. Etching solution in one chamber is in contact with the wafer&#39;s back surface while deionized water in the other chamber contacts the front surface. The relative liquid pressures in the chambers is arranged to be slightly higher in the chamber of the front surface so that leakage of etchant through a pin hole from back surface to front surface does not occur. As a further precaution, a monitor to detect the etchant is located in the DI water so that, if need be, etching can be terminated before irreparable damage is done.

FIELD OF THE INVENTION

[0001] The invention relates to the general field of microelectronicswith particular reference to forming thin membranes using corrosiveetches.

BACKGROUND OF THE INVENTION

[0002] Referring now to FIG. 1, bulk wet etching is a commonly usedprocess for creating a thin membrane (such as 14) or through-holes insilicon wafer 11 by using a hot alkaline etchant 13, such as KOH, TMAH(tetramethyl ammonium hydroxide), EDP (ethylene diamine pyrocatechol),etc. In this process, the wafer front side, where various devices and/orcircuits 17 are located, must be isolated from any contact with theetchant. However, due to non-uniformity of etching and/or defects of thebackside etching mask 12, it is not unusual, when the etching process isclose to its end, for the etchant to leak through perforations in thewafer (such as 15) to attack the front side (see 16). There is,therefore, a need in the bulk wet etching art to be able to ensure notonly etching uniformity but also leakage protection.

[0003] A number of methods and apparatuses have been invented toimplement wet etching of silicon wafers, including those thatincorporate various etch stop mechanisms, but little attention has beenpaid to the protection of wafers in case of leakage. Two typicaltechniques to prevent wafers from being attacked by etchant leakage areleakage detection and front side coating with etch resistant materials.

[0004] Leakage detection. This method uses electrodes or sensors todetect the presence of any alkaline etchant on the wafer front side.Once a leak is detected, the etching process is immediately aborted.However, in practice, it is found that leaks start from tiny pin holesand gradually spread over the wafer front side through capillary actionand surface tension forces. It is often too late by the time asufficient amount of leakage has occurred to be detected. This hasresulted in the loss/scrapping of a number of wafers in productionlines. In addition, aborting the etching process each time a leakagedetector is triggered can bring operational difficulties and risks ofwafer breakage.

[0005] Front side coating. In this approach, the wafer front side iscoated with material that is resistant to the etchant. After the etchingprocess, this coating is removed using an appropriate solvent. Typicalcoating materials are wax-based but some wax-based materials haveproperties that are incompatible with device processing or with thedevices themselves and are therefore not permitted in a clean room. Forexample, methyl benzene, the solvent for removal of black wax, may notbe used in most countries because it is hazardous to human health.Additional disadvantages reside in their cumbersome coating and removalprocessing. Furthermore, the coverage provided by the wax coating tendsto be inadequate when the wafer front side is non-planar, containingstepped layers of different heights. In these cases, the leaked etchantcan still spread out and attack the devices covered by the wax coatingthrough undercutting.

[0006] A routine search of the prior art was performed with thefollowing references of interest being found:

[0007] In U.S. Pat. No. 5,879,572, Folsom et al. show a bulk wet etchmethod while U.S. Pat. No. 5,338,416 (Mlcak et al.) shows a related etchprocess. Rolfson, in U.S. Pat. No. 6,025,278, shows a bulk wet etchprocess and apparatus in which the presence of etchant on the front sideof the wafer is monitored and used to terminate etching onceetch-through of via holes through the wafer has been achieved. Bydefinition, no steps can be taken to prevent etch-through (as is done inthe present invention). We note the following significant differencesbetween this reference and the present invention:

[0008] As noted above, Rolfson is for end-point detection only andprovides no protection to devices in the event of leakage. In thepresent invention the inert liquid (DI water) will dilute and remove anyetchant leak so that devices are protected during the brief periodbetween leakage detection and termination of etching.

[0009] In Rolfson, the inert and etchant liquids will exchange and/ormix at the end of wafer etching, whereas in the present invention thetwo liquids are made flow on the wafer surface for etching uniformityand leakage protection.

[0010] In Rolfson, the two liquids may be stagnant whereas in thepresent invention the etchant can be re-used since the inert liquid doesnot enter the etchant in case of leakage.

[0011] In Rolfson, neither liquid can be reused since they become mixed,thereby changing their properties.

[0012] In the present invention, liquids in the two chambers haveindependent flow rates and temperatures and maintain the same pressureon opposite sides of a wafer. This is not true for the Rolfsoninvention.

SUMMARY OF THE INVENTION

[0013] It has been an object of at least one embodiment of the presentinvention to provide an apparatus for etching the back surface of awafer while at the same time preventing any leakage of etchant to thefront surface.

[0014] Another object of at least one embodiment of the presentinvention has been to detect such leakage in the unlikely event that itoccurs.

[0015] Still another object of at least one embodiment of the presentinvention has been to ensure uniform etching of said back surface.

[0016] A further object of at least one embodiment of the presentinvention has been to provide a process for use with said apparatus.

[0017] These objects have been achieved by using two chambers that areseparated from each other by the wafer that is to be etched. Etchingsolution in one chamber is in contact with the back surface whiledeionized water in the other chamber contacts the front surface. Therelative liquid pressures in the chambers is arranged to be equal in thechamber of the front surface so that leakage of etchant through a pinhole from back surface to front surface does not occur. As a furtherprecaution, a monitor to detect the etchant is located in the DI waterso that, if need be, etching can be terminated before irreparable damageis done.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 illustrates how etchant may leak onto the front surface ofa wafer while its back surface is being etched.

[0019]FIG. 2 illustrates the apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The invention is a novel apparatus to make the wafer bulk etchingprocess an easy-to use, seamless and hassle-free procedure. In thedescription that follows the commonly used KOH etching process isassumed, but it will be realized that the invention is more general thanthat and could be applied to any situation where a material, such as anysemiconductor, is to be etched from one side to form etch-through holesor very thin membranes, while leaving an opposite side free of attack bythe etchant.

[0021] At the beginning of the etching process, hot KOH solution ispumped into the process chamber to etch the back side of the wafer. Inorder to have uniform etching across the wafer surface, the KOH flowpattern must be made turbulent. So, the KOH solution is ejected out atsome speed in random directions from its shower head. This turbulentflow also facilitates rapid removal of hydrogen bubbles generated by theetching reaction. An overflow hole on the upper part of the processchamber allows KOH to flow back the KOH tank. When etching is close tothe end (this can be estimated from the etching rate), hot DI water (atthe same temperature as the KOH) starts to be pumped into the enclosedchamber on the front side of the wafer. The water flow here is also madeturbulent such that no air bubbles accumulate and attach to the wafersurface. To avoid any damage to fragile parts of the wafer, the waterinjections from the showering should not be directed to the wafer. Afterflushing the wafer surface, the hot DI water is purged out through ashort tube to the water reflow tank and then flow back to the hot watertank. To balance the pressures on both sides of the wafer, the outlet ofthe short tube should be slightly lower in height that the overflow holeon the process chamber. In addition, the water flow should be kept at arelatively low rate so that the pressure difference to drive the waterflow is as low as possible.

[0022] The essence of the above scheme is to use clean water to diluteand take away any KOH leaks from the wafer front side. The purity ofwater can be monitored in the water flow tank by using a pH meter. Toohigh a pH value indicates either a large amount of KOH leakage or acontaminated wafer. In either of these case the etching process shouldnot continue any further.

[0023] Referring now to FIG. 2, seen there is a schematic diagram of theapparatus of the present invention. The heart of the apparatus is in thetwo chambers 21 and 22 that are separated from each other by siliconwafer 11 so that the latter's back surface 23 is part of chamber 21 andits front surface 24 is part of chamber 22. Seals 25 serve to preventliquid from flowing between these chambers. Generally, wafer 11 willhave a diameter between about 10 and 20 cm and a thickness between about0.5 and 0.72 mm.

[0024] A solution of potassium hydroxide 13 is stored in tank 27 whereit is maintained at a temperature between about 60 and 90° C. by heater26 controlled by thermocouple 28. Tank 27 is connected to chamber 21through pump 29 path, a return path being provided by etchant returnpath 30. The potassium hydroxide solution flow rate is between about 500and 2,00 sccm. The connection from pump 29 into chamber 21 terminates atshower head 31 that emits the potassium hydroxide solution in adirection that is parallel to back surface 23 while at the same timerotating. This is a key feature of the design since it inducesturbulence in the KOH solution which in turn ensures the rapid removalof any bubbles (primarily hydrogen) that form at back surface 23.

[0025] The main source of deionized water is tank 32 which is maintainedat a temperature between about 60 and 90° C. by heater 36 controlled bythermocouple 38. Tank 32 is connected to chamber 22 through pump 39. Theconnection from pump 39 into chamber 22 terminates at shower head 41that emits deionized water in a direction that is parallel to frontsurface 24 while at the same time rotating. This is also a key featureof the design since it induces turbulence in the DI water which in turnensures the rapid removal of any bubbles (primarily air) that may format front surface 24.

[0026] Deionized water leaves chamber 22 through path 40 but does notreturn directly return tank 32. Instead, it is connected to water reflowtank 42 which provides the means for introducing a difference inpressure between the two liquids that are on opposite sides of thewafer. This is accomplished by causing liquid in water reflow tank 42 toleave through high flow impedance tube 45 that extends for some distanceinto tank 42. The height of the KOH solution above the wafer should befrom 5 to 10 cm. The hydrostatic pressure of the KOH solution at thewafer surface can be balanced by that of the deionized water at thewafer surface. This balance is readily accomplished by raising orlowering tank 42 relative to 22.

[0027] A third important feature is the presence in chamber 22 ofdetector 48 for potassium hydroxide. While in principle this could beany suitable detector, in practice it is most convenient to employ a pHmeter for detector 48. In general, etching is terminated whenever the pHmeasured by 48 said pH meter exceeds 7.

[0028] Also shown in FIG. 2 are stirrers 51 whose purpose is to createuniform heating.

RESULTS

[0029] Prior to the adoption of the process and apparatus describedabove, the wafer yield of KOH etching, in terms of the wafers free fromKOH leak attack, varied from 0 to 70%. After implementation of thepresent invention, the wafer yield increased to 100%.

[0030] A comparison between the present invention and the prior art issummarized in TABLE I below: TABLE I LEAK FEATURE DETECTION WAX COATINGINVENTION Closed flow ckt. yes no yes Leak sensors yes yes no waxremoval no yes no wafer protection poor fair excellent processinterrupts highly likely no no operational ease clumsy clumsy easysystem complexity complex simple complex

What is claimed is:
 1. An apparatus for wet etching a wafer having frontand back surfaces, comprising: first and second chambers separated onefrom another by said wafer whereby said back surface is part of saidfirst chamber and said front surface is part of said second chamber; insaid first chamber, contacting said back surface at a first pressure, afirst liquid that etches said wafer; and in said second chamber, asecond liquid located at a height, that does not etch said wafer,contacting said front surface at a second pressure that equals saidfirst pressure, whereby leakage of said first liquid to said frontsurface, through a pin hole in said wafer, cannot occur.
 2. Theapparatus described in claim 1 wherein said first pressure can bebalanced by adjusting said height of the second liquid.
 3. The apparatusdescribed in claim 1 wherein said wafer has a diameter between about 10and 20 cm.
 4. The apparatus described in claim 1 wherein said wafer hasa thickness between about 0.5 and 0.725 mm.
 5. An apparatus for wetetching a silicon wafer having front and back surfaces, comprising:first and second chambers that are separated from each other by saidsilicon wafer whereby said back surface is part of said first chamberand said front surface is part of said second chamber; seals to preventliquid from flowing between said chambers; a first tank containing asolution of potassium hydroxide maintained at a first temperature; saidfirst tank being connected to said first chamber through a first pumpingpath and directly connected to said first chamber through an etchantreturn path; a second tank containing deionized water maintained at asecond temperature; said second tank being connected to said secondchamber through a second pumping path; said second chamber beingdirectly connected to a water reflow tank which is connected and to saidsecond tank through a water return path; means for introducing a balancein pressure between liquid contacting said back surface and liquidcontacting said front surface; and in said second chamber, a detector ofpotassium hydroxide.
 6. The apparatus described in claim 5 wherein saidmeans for introducing a difference in pressure between liquid contactingsaid back surface and liquid contacting said front surface furthercomprises means to control water height in said water reflow tankrelative to water height in said second chamber.
 7. The apparatusdescribed in claim 5 wherein said detector of potassium hydroxide is apH meter.
 8. The apparatus described in claim 5 wherein said firstpumping path terminates inside said first chamber at a shower head thatemits potassium hydroxide solution in a direction that is parallel tosaid back surface.
 9. The apparatus described in claim 8 wherein saidshower head rotates while it emits potassium hydroxide solution intosaid first chamber, thereby inducing turbulence.
 10. The apparatusdescribed in claim 5 wherein said second pumping path terminates insidesaid second chamber at a shower head that emits deionized water in adirection that is parallel to said front surface.
 11. The apparatusdescribed in claim 10 wherein said shower head rotates while it emitsdeionized water into said second chamber, thereby inducing turbulence.12. The apparatus described in claim 5 further comprising a sectionhaving high flow impedance in said water reflow path to reduce deionizedwater flow rate.
 13. The apparatus described in claim 5 wherein saidfirst temperature is between about 60 and 90° C.
 14. The apparatusdescribed in claim 5 wherein said second temperature is between about 60and 90° C.
 15. The apparatus described in claim 5 wherein deionizedwater flows into and out of said first chamber at a flow rate of betweenabout 10 and 100 sccm.
 16. A process for wet etching a wafer havingfront and back surfaces, comprising: at a first pressure, contactingsaid back surface with a first liquid that etches said wafer; at asecond pressure that is equal to said first pressure, contacting saidfront surface with a second liquid that does not etch said wafer,whereby leakage of said first liquid to said front surface does notoccur; and etching said wafer for a period of time without any part ofsaid front surface getting etched.
 17. The process described in claim 16wherein said wafer is a semiconductor.
 18. The process described inclaim 16 wherein said wafer has a diameter between about 10 and 20 cm.19. The process described in claim 16 wherein said wafer has thicknessbetween about 0.5 and 0.725 mm.
 20. The process described in claim 16wherein said first pressure can be balanced by said second pressure. 21.A process for wet etching a silicon wafer having front and backsurfaces, comprising: providing first and second chambers that areseparated from each other by said silicon wafer whereby said backsurface is part of said first chamber and said front surface is part ofsaid second chamber; providing seals that prevent liquid from flowingbetween said chambers; causing a solution of potassium hydroxide, at afirst temperature and pressure, to flow into and out of said firstchamber, in a direction parallel to said back surface through a rotatingshower head, whereby turbulence is induced in said potassium hydroxidesolution thereby removing from said back surface any bubbles that mayform there; causing deionized water, at a second temperature andpressure, to flow into and out of said second chamber in a directionparallel to said front surface through a rotating shower head, wherebyturbulence is induced in said deionized water thereby removing anybubbles that may form on said front surface; monitoring deionized waterin said second chamber for the presence of potassium hydroxide; andetching said back surface for a period of time that is insufficient forsaid potassium hydroxide solution to etch all the way through any partof said silicon wafer.
 22. The process described in claim 21 whereinsaid solution of potassium hydroxide has a concentration between about30% and 40% by weight.
 23. The process described in claim 21 whereinsaid first temperature is between about 60 and 90° C.
 24. The processdescribed in claim 21 wherein said second temperature is between about60 and 90° C.
 25. The process described in claim 21 wherein the step ofmonitoring deionized water in said second chamber for the presence ofpotassium hydroxide further comprises measuring the pH.
 26. The processdescribed in claim 25 further comprising terminating etching when saidpH exceeds 7.